"Radomes" an acronym for radar domes are employed as shields for protecting antennas against harsh weather conditions and other environmental factors. Properly designed radomes are not only protective, but highly "transparent", since the performance of the antenna must be consistent and predictable regardless of variations in environmental conditions.
Since antennas used for radar and satellite communication are inevitably complex and large structures, the radome itself is also correspondingly large and, for this reason, is generally constructed by assembling a large number of panels. Various techniques for assembling radomes are known. One known type of radome employs a metal space frame (MSF) formed of a triangular lattice of metal struts, across which a thin material is stretched and which is transparent to the radar or similar signal broadcast to or from the antenna. The lattice is formed of aluminum struts whose quasi-random distribution with respect to the spherical envelope reduces the effects of interference to the radiation by the metallic lattice. On average, the loss associated with such a structure is between 0.5 to 1 dB and the energy distribution to the sidelobes is substantially random, resulting in amplification of the side lobes.
A second, more advanced, type of radome is formed of concave panels having a sandwich structure. The panel itself is made from a foam core surrounded by two fiberglass skins. The skins are pre-impregnated fiberglass-reinforced polyester laminates, with a closed-cell polyurethane foam core. The sandwich construction tapers to a solid laminate joint that may be tuned for optimal performance. The basic geometry is based on an icosahedron that is further subdivided and distorted in order to provide quasi-random dispersion.
U.S. Pat. No. 5,344,685 (Cassell) discloses a contoured sandwich radome having a core consisting of a series of flexible foam mandrels laid side-by-side, with inner and outer facings made up of a fabric impregnated with a resin system, capable of providing superior breakdown voltage characteristics when cured in one stage. Such structures are obtained by a process comprising laying a plurality of first plies of resin impregnated fabric in a contoured mold to form an outer facing. A plurality of narrow straight but flexible mandrels of plastic foam is provided, individual groups of such foam mandrels being laid in side-by-side relation on to the outer facing. One series of the foam mandrel groups is positioned in a substantially transverse direction in the mold and another series of the foam mandrel groups is positioned in a substantially longitudinal direction in the mold. The inner ends of the mandrel groups abut the sides of adjacent mandrel groups, to form a foam core. The mandrels which are essentially straight before being installed in the mold are kept from springing up from the mold contour by a restraining collar which the outer ends of the mandrels abut, the collar being removed after the mandrels have been pressured under vacuum. A plurality of second plies of resin impregnated fabric are laid over the foam core, and the plies of resin impregnated fabric are cured to form the inner and outer facings.
U.S. Pat. No. 5,357,726 (Effenberger et al.) discloses a flexible, reinforced textile composite material for constructing tensioned fabric structures having particular application, inter alia, as a radome.
U.S. Pat. No. 5,323,170 (Lang) discloses a radome constructed using a rigid or semi-rigid foam core and sandwich construction. The foam core radome has improved water rejection properties and also provides greater impact strength and consistent, high radar transparency without sacrificing weight and structural stability, thereby providing a much longer service life in moisture/impact critical environments. Preferably, the radome includes a vinyl rigid closed-cell foam core consisting of a polymeric alloy of a cross-linked aromatic polyamide urea and a linear vinyl polymer.
One of the main advantages of a radome having a sandwich construction over that based on the MSF is inherent in the possibility to tune electrically the frameworks. This is done by embedding printed circuits of the metallic networks within the fiberglass of the framework. These circuits are correlated to a specific frequency (within the working frequency range of the antenna) and greatly enhance the general transmission efficiency of the radome even to as much as 98% (less than 0.25 dB drop). On account of the correlation, a lower quantity of energy is dispersed from the framework (in certain cases less than -50 dBc relative to the peak radiation).
Such tuning of the framework has been achieved in the frequency range between the L-Band and the C-Band. At higher frequencies it is difficult to realize the tuning on account of the need to embed a large number of layers of printed circuits (in accordance with the C-Band three such layers are required). At even higher frequencies (Ku-Band and higher) the sandwich boards themselves are not sufficiently transparent. FIG. 1 shows graphically the frequency response of typical sandwich type radomes for different angles of incidence from which a noticeable drop in average transmission is seen to between 0.1 and 0.2 at frequencies between about 8 and 14 GHz.
These drawbacks of sandwich radomes do not become apparent so long as the radomes are intended for air controlled radar antennas and weather radar antennas. However, once the use of radomes is contemplated for ground based stations for satellite communications, such radomes are no longer suitable. In order not to interfere with neighboring satellites, the ground station antennae are required to withstand the stringent requirements of CCIR which limit the side lobes to less than the envelope of 29-25*log(.theta.) dB, or to other similar envelopes, in accordance with the class of ground station. Any deviation from this specification renders the station unfit such that it will not obtain a valid operating license.
The high scattering from MSF radomes gives rise to side lobes which, in the best case, are on the borderline of the specification. Consequently, those who install such stations are prevented from using radomes, even when the climatic conditions are extreme and protection of the antenna by means of a radome would lead to substantial operational savings and improved maintenance of the station. All of this arises from the danger of non-conformance with the CCIR standards. It might just be possible to conform sandwich radomes to ground based antenna stations which are operational in the C band i.e. 4 to 6.5 GHz for which panel transmission is high as shown in FIG. 1, although on the face of it, there is no guarantee that the radome will have sufficient bandwidth owing to possible bandwidth limitations of the tuned joints. However, it is very difficult to operate such an arrangement at both the C band and the Ku band (10.7 to 14.5 GHz) since panel transmission is not high for both frequencies simultaneously and this bears a heavy penalty in the reduction of the radome's strength.
Tuned solid wall radomes are very common in airborne, missile, naval and even small single unit ground based radomes. However, they have not been used in large segmented ground based radomes which must have tuned walls demanding tight tolerances of the wall thickness and are, therefore difficult to achieve in large sizes. Solid wall, large ground based radomes have thus only been implemented with "thin walls" wherein the wall thickness is very small compared with the operating wavelength such that they are substantially insensitive to variations in wall thickness.
Radome manufacturing processes that offer good thickness control require double molds and are usually performed at high pressure. Thus, the molds must be capable of withstanding high pressures and are therefore expensive and necessarily massive structures. There are two processes that allow manufacture of large parts at low pressure: namely, the use of Resin Transfer Molding (RTM) and the other being thermal forming of thermoplastic materials. RTM is an excellent candidate for narrow band applications wherein the high dielectric constant of the material limits the possible bandwidth of a tuned wall radome. Thermoplastic materials have much lower dielectric constants and are therefore suitable for wide band or multi-band applications such as dual band satellite communication radomes.
It will further be borne in mind that radomes for ground based antenna systems are large structures, typically several meters in diameter and must be able to withstand wind velocities well in excess of 150 mph (240 Km/h). At the same time, they must, of course, be substantially transparent to the signal which is transmitted to or from the antenna.
Most of the above references are directed to low frequency and/or narrow band applications are therefore suffer from the specified shortcomings when used for high frequency or wide band applications. Whilst, MSF radomes are suitable for such applications they suffer from high scattering sidelobes.